In recent years, with the rapid spread of IT-related devices and communication devices such as personal computers, video cameras and cellular phones, emphasis is placed on the development of batteries that can be used as the power source for such devices. In the automobile industry, the development of high-output and high-capacity batteries for electric vehicles and hybrid vehicles has been promoted. Among various kinds of batteries, a lithium battery attracts attention due to its high energy density and power output.
In a lithium battery, generally, a lithium metal complex oxide having a layered structure, such as lithium nickelate or lithium cobaltate, is used as the positive electrode active material, and a carbonaceous material that is able to occlude/release lithium ions, a lithium metal, a lithium alloy or the like is used as the negative electrode active material. As the electrolyte disposed between the positive and negative electrodes, a liquid electrolyte in which a lithium salt is dissolved, a lithium-containing solid electrolyte or the like is used.
As described above, lithium batteries are excellent in energy density and power output. On the other hand, they have bottlenecks such as an increase in lithium prices with an increased demand for lithium batteries, and limited lithium reserves, thus preventing them from entering into mass production or making it enlarged.
Therefore, studies of sodium batteries are in progress, in which sodium, which is a plentiful resource and low in cost, is used in place of lithium. For example, Patent Literature 1 discloses a positive electrode active material for non-aqueous electrolyte secondary batteries, which is represented by MaxMbyP2O7 (Ma is Na, Li, Ca or Mg; Mb is a transition metal being stably present at tetravalent or higher valent condition; x is a value that satisfies 0≦x≦4; y is a value that satisfies 0.5≦y≦3; and z is a value that satisfies 6≦z≦14). In Patent Literature 1, the positive electrode active material which was actually produced and evaluated under “Examples” is MoP2O7.